Recovery of Rare Earth Elements from Coal Ash with a Recycling Acid Leach Process

Rick Peterson1, Mike Heinrichs1, Justin Glier1, Annie Lane1, Rachid Taha1 1Battelle Memorial Institute, Energy Resource Group, 505 King Avenue, Columbus, OH 43201 CONFERENCE: 2017 World of Coal Ash (www.worldofcoalash.org) KEYWORDS: rare earth elements, coal ash, coal liquefaction

1.0 INTRODUCTION Rare earth elements (REE) have a wide range of uses in catalysis, alloying, magnets, and optics, and have become critical for the batteries, motors, and generators that enable sustainable technologies and infrastructure. Despite these critical applications, the United States has sustained very little in the way of REE resources, with China producing more than 80% of the global supply of the elements1. A technology that will allow for the economical recovery of REEs from coal will reduce dependence on foreign supplies and ensure US capability for renewable and defense products.

Rare earth elements have been found in varying concentrations ranging up to 1,000 parts per million by weight in the following materials in the United States: coal mine roof and floor materials, run-of-mine coal, prepared coal, partings, pit cleanings, coal preparation refuse, and tailings. REEs can be found in coal byproducts, including ash, coal-related sludge, and mine drainage. Certain coals can contain a higher ratio of heavy (generally more valuable) REEs than found in other sources of REEs such as natural ores. Given the potentially low REE concentrations in the feed materials, and subsequent potentially low yield of REEs from any separation process, minimizing extraction costs is a key challenge.

Battelle aims to validate the economic viability of recovering REEs from coal ash using its patented (US6011193) closed-loop Acid Digestion Process (ADP). This validation will be accomplished by selecting a source of coal ash that consistently provides concentrations of rare earth elements above 300 parts per million by weight and in a form suitable for leaching. In support of this study, Battelle conducted a sampling and characterization study in which ash from power generation stations, low temperature combustion coal ash, and ash from Battelles coal liquefaction process were assessed as potential sources of REEs. Next, Battelle conducted an economic feasibility study 1 U.S. Geological Survey. (2017, January 19). Mineral Commodity Summaries 2017. Retrieved

from minerals.usgs.gov: https://minerals.usgs.gov/minerals/pubs/mcs/2017/mcs2017.pdf

2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, 2017http://www.flyash.info/

focused on REE recovery from ash sourced from a power generation station, as ash sourced from power generation facilities is much more readily accessible than other sources of ash. The assumptions and economic sensitivities in the process that are used in a Technoeconomic Analysis (TEA) and certain required design parameters directed laboratory testing to validate the TEA and allow design of a bench-scale system to prove the process on a continuous basis. Additionally, Battelle created a CHEMCAD model which simulated the recovery process at the 30 tonnes per hour scale proposed by the TEA.

The sampling and characterization results presented in this paper inform the process economics and future process design by assisting in the selection of key materials to target for recovery. These results, combined with the TEA, form a feasibility assessment, in which the economic feasibility of recovering REEs from coal ash is evaluated. Current market pricing information was collected for each of the REEs, as well as a CHEMCAD model which simulated the proposed recovery process. Scenarios in which economical recovery of REEs are possible are presented in the TEA results.

2.0 SAMPLING AND CHARACTERIZATION STUDY Battelles approach included characterization of ash from pulverized coal combustion plants, a fluidized bed combustion plant, and a direct coal liquefaction process, as illustrated in Figure 1. High temperature ash is typical of pulverized coal combustion power plants, where it is heated to temperatures in excess of 1,200C in the furnace. This high temperature creates a vitrified, glassy ash particulate that may entrap rare earth elements in regions that are not amenable to acid leaching. Lower temperature ash can be obtained from fluidized bed combustion facilities, which operate at lower furnace temperatures near 800C, and should produce ash with less vitrification that is more easily leached. The direct coal liquefaction ash is not exposed to high temperature oxidation, and the REEs should be near the mineral form in which they are found in the raw coal. The coal liquefaction ash was further processed to divide it into sections by density and by particle size to determine if there is a simple mechanical process that can be used to enrich the REEs concentrations.

Figure 1: Schematic describing coal ash samples of interest in this study.

High temperature ash and low temperature ash were obtained from coal combustion power plants in the Appalachian Basin. Fly ash and bottom ash were collected from all of the plants, but feed coal was only collected at some of the plants. There is not a direct association between the feed coal and the ash samples that were collected, since in many cases the plant operations included significant ash and coal storage, but the samples were taken as near to the production point as feasible. It was not feasible to obtain fly ash at different points in the flue gas treatment train since this handling is largely automated, and safe access to the ash could not be provided until after the fly ash sources were combined. High temperature ash samples were obtained from pulverized coal plants, and low temperature ash samples were obtained from a fluidized bed coal combustor. Liquefaction ash was taken from pilot runs of Battelles direct coal liquefaction process.

2.1 Sample Locations Data from the US Geological Survey (USGS) was used in conjunction with databases from the West Virginia Geological and Economic Survey (WVGES) and Pennsylvania Geological Survey (PAGS) to identify coal samples locations that may have high REE concentrations. A map of approximate sample REE values in the area of study is shown in Figure 2. WVGES also provided access to its extensive coal sample inventory; two

Figure 2: Map of Kentucky, Ohio, Pennsylvania, and West Virginia with the worth of coal ash labeled based on USGS COALQUAL data and standardized pure rare earth metal prices.

coal samples with high expected REE value were obtained based upon prior USGS and WVGES analyses. These were samples 11250 and 13423 in the WVGES database. A standard table2 for pure rare earth metal prices was used to assign approximate ash worth throughout this report unless noted otherwise.

In the initial examination of the USGS database, Kentucky coal was found to have generally higher REE values on an ash basis than neighboring states, as shown in Figure 2. The Kentucky Geologic Survey (KGS) provided an explanation for these high concentrations and identified the Fireclay coal seam as a target formation for this study. The Middle Pennsylvanian Fireclay coal seam has a unique layer of tonstein, a layer of ash deposited in a coal forming swamp. This volcanic ash overburden increases the concentration of REEs in the coal through multiple leaching and transport methods.

Power plant samples were obtained with the support of commercial partners to find facilities that were representative of coal power stations. The low temperature sample was obtained by contacting an operator of a known fluidized bed combustion power plant. Coal liquefaction ash was generated at Battelle and used Middle Kittanning coal from Ohio due to its availability and known REE concentration above 300 ppm.

2 http://mineralprices.com/ (Accessed 14 April 2016)

2.2 Results 2.2.1 High Temperature (Pulverized Coal Combustion) Samples All samples were digested by either a sodium peroxide or lithium metaborate/ tetraborate fusion, dissolved in nitric acid, then analyzed for elements by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Pulverized coal combustion (PCC) ash was obtained from four operating power plants. Both bottom ash and fly ash samples were collected, and in one case feed coal samples were also obtained. A summary of the REE content for all of the PCC plant samples is shown in Table 1. PCC Plant A was unique in that it had two separate feed coal piles feeding different units. Feed Coal A was generally washed and of higher quality than Feed Coal B and had higher REE content. However, the feed coal samples did not originate from the same mine(s), and so no conclusions can be drawn about the effect of washing on coal ash REE concentration. The fly ash collected from PCC Plant A was associated with Feed Coal A, and it had the highest REE+Y+Sc concentrations, as well as attractive Heavy Rare Earth Element (which are typically more scarce and more valuable) to Light Rare Earth Element (HREE/LREE) ratios. It was accordingly selected for replicate analyses to understand variability in the analytical methods as well as build confidence in the starting concentration of REE for later leaching tests. Two samples of this fly ash were taken. Sample 1 came from a single truckload of ash, while Sample 2 was collected from four separate truckloads. It should be noted that Yttrium and Scandium are commonly considered REEs along with the lanthanide metals due to their similar chemical characteristics, and Yttrium is commonly grouped with heavy REEs, while Sc is grouped with light REEs.

Table 1: Summary o